Researchers have determined that the mouth’s microorganisms have evolved along with the human species, probably for as long as it has existed. In exchange for living in their tropical paradise, the mostly beneficial bacteria help fend off disease-producing germs that attempt to infiltrate the mouth from the outside world. For example, some beneficial bacteria produce organic acids, such as proprionic and buteric acid, that kill organisms responsible for a number of intestinal problems.Yet when human babies pop into the world, their wails of greeting burst from sterile mouths. “Within minutes to hours, however, they are colonized with organisms that stay with them until they die,” says Socransky. These bacteria, yeast, viruses, and protozoa, most of which are harmless, enter from anything that makes contact with a baby’s mouth: air, breast, bottle nipple, thumb, and other objects.
The growth of organisms in the mouth follows the classical pattern of ecological succession-the way bare land eventually turns into thick jungle. A few pioneer species settle first, creating a habitat friendly to other species, which then, too, move in. When the first baby teeth push through the gums, another set of species-including the dreaded Streptococcus mutans, the bacteria believed responsible for most tooth decay-takes hold. During puberty, the composition of saliva changes, so that still another group of organisms immigrate and flourish. By the time humans reach adulthood, their mouths harbor what’s known as a climax community-a complex group of organisms, each with its own preferred microhabitat.
Although diets vary the world over, dental researchers have found the same organisms in human mouths no matter where people live. Some species live only on the cheeks. Others prefer the back of the tongue versus the front, especially the group of anaerobic bacteria that live in the crevices of the tongue and emit hydrogen sulfide, the origin of most severe bad breath. Another group will survive only on the palate. And the teeth themselves provide a plethora of living options-surfaces open to the outside world, sides facing the back of the mouth, a strip along the edge of the gums, and the gloomy, wet, oxygen-deprived spaces between the gums and the teeth.
Saliva, the amazing fluid that keeps this ecosystem in balance, harbors its own collection of bacteria, as well as a host of other substances. Bicarbonate ions buffer the tooth-decaying acids produced by harmful bacteria such as S. mutans. Phosphate and calcium ions supersaturate saliva and continually repair the microscopic chinks made in the teeth by the bacteria’s acid.
Saliva also contains antibacterial agents, such as lysozyme, which kill bacteria by opening up their cell walls. About 60 proteins float around in saliva. Some of them actually provide nutrients for bacterial growth while others lubricate the mouth and cause bacteria to stick together in such large clumps that they can’t adhere to tooth surfaces and are easily washed away. Saliva even contains antiviral components. In fact, researchers at the National Institute for Dental Research, who have found that the AIDS virus does not live in saliva, are trying to isolate a substance that they believe may be effective against the AIDS virus.
Most of the time, the inhabitants of the mouth live in more or less perfect harmony. “Congress should take lessons from the mouth,” says Yolanda Bonta, manager of clinical research at Colgate Oral Pharmaceuticals in Piscataway, N.J. But, like breakdowns in budget talks that sometimes lead to government shutdowns, sometimes things go to hell in a handbasket in the mouth, too.
Indeed, ecological conditions in the mouth are never stable. People change their diets, lose teeth, have crowns or false teeth put in, or take drugs that affect certain microorganisms. “For example, an epilepsy medication causes overgrowth of gums,” says Socransky, “and that changes the microbiota.” Radiation of the head and neck for cancer treatment as well as a host of medications also cause a drastic drop in saliva production, allowing bacteria to run rampant.
Changes also occur after every meal, after every brushing and flossing, even every time we swallow, as millions of bacteria lose their grip on tooth surfaces and tumble down the throat. During a night’s sleep, when saliva production drops to near zero, bacteria, like the minions in the “Fantasia” version of Moussorgsky’s “A Night on Bald Mountain,” revel in their freedom and multiply with abandon until the dawn.
Abundant sugar can power S. mutans into a frenzy of activity. In fact, while some strains of S. mutans produce natural antibiotics against the bacteria that cause strep throat, and were probably useful in the mouths of people of primitive cultures, the preponderance of refined sugars in the modern diet changed the oral landscape so drastically that S. mutans is now more harmful than helpful. As it gobbles up sugar, S. mutans produces much more acid than saliva can buffer, and the excess eats away at the minerals of tooth enamel. Without adequate brushing and flossing, plaque grows, producing calcified deposits and a cozy home for more species that do more damage. Sticky opportunistic bacteria grab hold in the newly formed holes and crevices, causing tooth decay, and no amount of salival flow will wash them off.